The ability to pinpoint the location of mobile terminals is a desirable feature of a mobile telephone network. This is because of the need to provide customer services which rely on knowing the whereabouts of users of these services. For example, up-to-date local traffic information can be provided to enable a user to avoid nearby traffic jams. A user may also wish to know, for example, how to get to the nearest pub or restaurant from their present location. Clearly the location of the user must be ascertained to within even a few metres for this type of service to work.
Another reason for wishing to know the location of a mobile terminal is so that emergency services can locate a caller who is unable to provide an accurate personal location themselves.
It is known in a GSM mobile network to provide the location of a mobile telephone in terms of the cell of the network in which the telephone is located. Each cell contains one base station and a telephone is only ever in traffic communication with one base station at a given time. Hence the location of the telephone to an accuracy of the cell area can be determined simply by ascertaining with which base station the telephone is communicating. Such methods are known as cell-based location methods. Other methods can be combined with the cell identity (CI), such as a triangulation system, in which the location of a particular mobile phone is calculated using control signals from at least the three base stations closest to it (two of which are located in adjacent cells to the cell in which the mobile telephone is located). This system uses the assumption that the distance of the phone from a base station is proportional to the strength of the signal which the base station receives from it, or the time taken for the signal to travel between the phone and the respective base station. Thus the position of the phone can be determined by comparing the relative strengths or travel times of received signals between the three base stations and thus assessing the distance of the user from each base station. The actual location of the user is then obtainable geometrically since the location of the base stations is known and fixed.
In a 3GPP (3rd Generation Partnership Project) network using a Wideband Code Division Multiple Access (W-CDMA) signalling system, it is possible for a mobile terminal to be in active communication with more than one base station at any one time. This situation is known as “soft handover” and differs from (hard) handover in a GSM system, in which a mobile terminal is “handed over” from one base station to another as it moves between cells of the network. Because of the nature of the soft handover, the above-described cell-based mobile location procedures suitable for GSM can not always be used in a W-CDMA type signalling system. It has therefore been necessary to develop more reliable ways of locating a mobile terminal in this type of signalling system.
In W-CDMA a “softer handover” is defined as well. In the case of “softer handover” the antennas of the base stations with which the mobile station is communicating are co-located (e.g. they are installed at the same physical location or site). In the remainder of this document, the term “soft handover” will be used also to cover the case of “softer handover”, and it will be understood by those skilled in the art that the invention and the described embodiments thereof are applicable to a softer handover situation as well as a soft handover situation.
Location techniques are known which are directed towards calculation of a location estimate and a “confidence region” associated with the location estimate of a User Equipment (UE).
One such technique is a Cell Identity and Round Trip Time (CI+RTT) location method.
The CI+RTT location method in UMTS relies on the availability of Round Trip Time (RTT) and Rx−Tx Time Difference (RxTxTD) measurements. RTT and RxTxTD measurements are introduced in UMTS FDD (Frequency Division Duplex) to allow the implementation of the CI+RTT location method.
The RTT is defined as RTT=TRxUL−TTXDL, where TDTXDL is the time of transmission of the beginning of a downlink dedicated physical channel (DPCH) frame to a User Equipment (UE) and TRXUL is the time of reception of the beginning (the first detected path, in time) of the corresponding uplink DPCCH (Dedicated Physical Control Channel)/DPDCH (Dedicated Physical Data Channel) frame from the UE.
The RTxTxTD=TTxUL−TRxDL is the difference in time between the UE uplink DPCCH/DPDCH frame transmission (TTxUL) and the first detected path (in time) of the downlink DPCH frame from the measured radio link (TRxDL).
RTTs are measured by the base stations, RxTxTDs are measured by the UE.
By combining a pair of RTT and RxTxTD measurements referred to the same base station the distance between the UE and that base station can be estimated. Such distance estimate is analogous to the distance estimate that can be obtained from one Timing-Advance (TA) in GSM. In this sense, the CI+RTT location method corresponds to the Cell Identity+Timing Advance (CI+TA) location method in GSM. However, two particular features of UMTS FDD make the CI+RTT method potentially more accurate than the CI+TA method in GSM:    1. The much shorter UMTS chip period as compared to the GSM bit period affects the resolution with which a distance estimate can be determined from a TA in GSM or from an (RTT, RxTxTD) pair in UMTS. One GSM bit period is equivalent to approximately 1100 meters while one UMTS chip period is equivalent to approximately 80 meters, thus the distance measurements resolution in UMTS is better than in GSM.    2. In UMTS a UE can be in soft handover. UMTS standards require that RTTs and RxTxTDs are measured for each active radio link, thus multiple distance estimates can be potentially available for locating one UE in UMTS. In GSM this is not possible since the TA is available only for the unique serving cell.
In the CI+RTT location method the unknown geographical coordinates of the UE whose position it is desired to determine are estimated by combining absolute distance measurements between the UE and the base stations in the active set. Each absolute distance measurement is calculated from each (RTT, RxTxTD) pair.
Existing CI+RTT location methods can be considered to fall broadly into two general categories, single-site location methods and multi-site location methods. There are a number of good, reasonably accurate algorithms for implementing multi-site location methods when the UE have active radio links with cells located at three or more sites. However, such methods often fail when two or less sites are available for location calculation purposes.
One of the difficulties with existing location systems is that a location system method is selected to generate a location estimate which may then fail for one or more reason. It then becomes necessary to try a different method, which may or may not be implementable at the site where the first location method was carried out.
It is an aim of the present invention to obviate or at least mitigate this difficulty.